(a) Banks can be vague in their SAR filings with “transaction with no apparent economic, business or lawful purpose”
(b) What BSA manul says: "No bank, and no director, officer, employee, or agent of a bank that reports a suspicious transaction may notify any person involved in the transaction that the transaction has been reported." [1]
This deadly combination is enough for banks to NOT disclose. That's why a vague reason is provided by banks; here is Chase's template: "Financial institutions have an obligation to know our customers and monitor transactions that flow through our customers' accounts. After careful consideration, we decided to close your account because of unexpected activity on these or another Chase account."
[1] https://bsaaml.ffiec.gov/manual/AssessingComplianceWithBSARe...